The invention relates to front end accessory drives, and more particularly to belt drive systems having an asymmetric damping tensioner.
Most engines used for automobiles and the like include a number of belt driven accessory systems which are necessary for the proper operation of the vehicle. The accessory systems may include an alternator, air conditioner compressor and a power steering pump.
The accessory systems are generally mounted on a front surface of the engine. Each accessory has a pulley mounted on a shaft for receiving power from some form of belt drive. In early systems, each accessory was driven by a separate belt that ran between the accessory and the crankshaft. Due to improvements in belt technology, single serpentine belts are now generally used in most applications. A single serpentine belt routed among the various accessory components drives the accessories. The engine crankshaft drives the serpentine belt.
Since the serpentine belt must be routed to all accessories, it has generally become longer than its predecessors. To operate properly, the belt is installed with a pre-determined tension. As it operates, it stretches slightly over its length. This results in a decrease in belt tension, which may cause the belt to slip. Consequently, a belt tensioner is used to maintain the proper belt tension as the belt stretches during use.
As a belt tensioner operates, the running belt may excite oscillations in the tensioner spring. These oscillations are undesirable, as they cause premature wear of the belt and tensioner. Therefore, a damping mechanism is added to the tensioner to damp operational oscillations.
Various damping mechanisms have been developed. They include viscous fluid dampers, mechanisms based on frictional surfaces sliding or interaction with each other, and dampers using a series of interacting springs. For the most part these damping mechanisms operate in a single direction by resisting a movement of a belt in one direction. This generally resulted in undamped vibrations existing in a belt during operation as the tensioner arm oscillated between loaded and unloaded positions.
The prior art systems rely on a tensioner set up to be compliant in order to follow the motion of the belt. Usually the tensioner is set up with a low damping rate to facilitate this compliance. As a result the prior art systems operated in an unsatisfactory manner during load changes. The accessory drive operated normally when the engine was running at a steady RPM. The tensioner bearing against the belt would maintain a tension in the span. Generally, the tensioner is xe2x80x98downstreamxe2x80x99 of the crankshaft in a belt movement direction. Damping was set so that the tensioner would damp most of the vibrations in the running belt.
The problems arise when the engine speed is rapidly changed, in the range of 5000 to 10000 RPM/sec. In this case, the accessories such as the alternator continue to drive the belt after a speed reduction due to rotational inertia. This causes the belt on the xe2x80x98downstreamxe2x80x99 side of the crankshaft to tighten, loading the tensioner. If the damping rate in the tensioner is too low the tensioner will be unable to resist the increase in belt tension and the arm will move in a direction away from the belt. As a result, the tensioner is not maintaining sufficient tension in the belt. This will allow the belt to slip on the crankshaft pulley, since the belt is now being driven toward the crankshaft, causing squeeking noises. Prior art systems rely on a means of locking the tensioner arm in the loading direction to prevent the decrease in belt tension. However, locking the tensioner prevents the tensioner from performing its corollary function of damping vibrations in the belt.
Representative of the prior art is U.S. Pat. No. 5,439,420 to Meckstroth et al. which discloses an accessory drive system including a tensioner having a governor for controlling rotational motion of the arm with the arm being able to rotate freely in the direction in which tension of the belt is increased and with the governor resisting motion of the arm in the direction in which tension in the belt is decreased.
The prior art also teaches a method of arranging engine accessories so that the order of rotational interial force is greatest for the accessory nearest the crankshaft pulley as seen from the tight side of the belt. This is taught in U.S. Pat. No. 4,959,042 to Tanaka. This method does not rely on the operational characteristics of the tensioner, instead relying on the dynamics of the staggered order of the accessories based upon rotational interia.
The prior art systems depend upon a locking tensioner or upon a particular mechanical arrangement to address the problem of high rate of change of engine speed. Neither system solves the dual problems of preventing squeal during speed changes while continuing to damp belt vibrations. Further, the prior art systems, in the case of Mechstroth are complex and expensive, requiring complex mechanical devices to control the movement of a tensioner arm. The prior art systems are relatively large requiring room on the engine surface. The Tanaka method does not fully address the issue of high deceleration rates, relying instead on the arrangement of the components which does not fully defeat the tightening of the belt during deceleration.
Reference is also made to co-pending U.S. patent application Ser. No. 09/861,338 filed May 18, 2001 which discloses a tensioner having a damping mechanism.
What is needed is an asymmetric damping tensioner belt drive system having an asymmetric damping tensioner. What is needed is an asymmetric damping tensioner belt drive system capable of providing a higher belt tension during rapid changes in engine speed. What is needed is an asymmetric damping tensioner belt drive system having a greater damping friction in a loading direction than an unloading direction. What is needed is an asymmetric damping tensioner belt drive system having a coefficient of asymmetry in excess of 1.5. The present invention meets these needs.
The primary aspect of the invention is an asymmetric damping tensioner belt drive system having an asymmetric damping tensioner.
Another aspect of the invention is to provide an asymmetric damping tensioner belt drive system capable of providing a higher belt tension during rapid changes in engine speed.
Another aspect of the invention is to provide an asymmetric damping tensioner belt drive system having a greater damping friction in a loading direction than an unloading direction.
Another aspect of the invention is to provide an asymmetric damping tensioner belt drive system having a coefficient of asymmetry in excess of 1.5.
Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings.
The invention comprises an asymmetric damping tensioner system for belt drives on an engine. A belt is connected between a driver pulley on a crankshaft and any number of driven pulleys. Each driven pulley is connected to an accessory such as an alternator, power steering pump, compressor or the like. The tensioner is placed anywhere before the first component of significant effective inertia, in the belt movement direction. A biasing member in the tensioner is used to maintain a tension in the belt. The tensioner further comprises a damping mechanism to damp belt vibrations caused by the operation of the engine. Tensioner damping friction is unequal or asymmetric, depending upon the direction of movement of the tensioner arm. During acceleration the damping friction of the tensioner in the unloading direction is significantly lower than the damping friction in the opposite, or loading direction, as is the case during deceleration. Lower damping friction during acceleration allows the tensioner arm to quickly adjust to the increase in belt length caused by acceleration. Higher damping friction during deceleration prevents the tensioner arm from being moved too far in the loading direction thereby causing slipping and noise. Asymmetric damping also significantly diminishes overall vibration in the belt during all phases of operation.